CI: remove --check-files and add conditional to prevent duplicate run

[thedavidprice]

Re-added --check-files, which was removed due to actions/cache infrequently giving a false positive "cache hit" on Windows runner. The problem is that the Windows runner has reverted back to taking > 10 min for each node version. There have been a couple of updates to actions/cache, so I'm curious if there's been any improvement on Windows.

Looking into the Windows long run time specifically, there is an issue with Yarn giving a warning "There appears to be trouble with your network connection. Retrying...", which seems to account for half of the runtime. There's a lot of conversation about this in Yarn Package GH Issues. This comment seems to be the most helpful as a potential next step to fix: yarnpkg/yarn#5259 (comment) I think it would require creating a .yarnrc file in root with the registry and HTTP changes suggested.

[Update]
Also added something I've been wanting to try --> a conditional so that the whole Action will not re-run when a PR is merged.

Problem: because this triggers on both Push and specific PR events, when a PR is merged, the action is effectively re-triggered. We need to have the Push trigger because there are some occasional instances when a direct push to main is needed.

Solution: I added the conditional if: github.event.pull_request.merged == false at the beginning of the jobs. This should allow a direct push to trigger the full runner. However, if the trigger is due to a PR merge event, then the action will trigger but the jobs will not run.

[peterp]

🚀

[thedavidprice]

Not sure if the conditional is working as is. Need to watch for Actions running full jobs process when PR is merged.